Due to their very large surface area, metal nanoparticles (MNPs), especially those of noble metals, such as platinum, palladium, ruthenium, rhodium and gold, are widely used as effective catalysts in various kinds of chemical processes. In most cases, MNPs are immobilized onto solid support materials or stabilized as colloidal solutions. The support materials are generally based on porous inorganic materials, such as carbon, silica, titania or alumina, in order to allow an easy access of all reactants to the catalyst surface. A common strategy to immobilize MNPs onto a support material is the impregnation method in which the support is immersed into a solution of a metal precursor, dried and calcined. After that, the metal is reduced by some reducing agent, often under harsh conditions, to form metal nanoparticles. However, it is difficult to control the particle size by this method, as the size distribution can be wide with particles beyond ten nanometers or more.
Catalytic materials based on MNPs onto support materials suffer of other problems as far as their reuse and engineering into reactors is concerned. Use of batch reactors in a two phase liquid system involves the recover of the catalyst from the reaction solution after reaction completion by appropriate methods, such as filtration and centrifugation. However, it is not easy to separate the catalysts, when they are in the form of fine powders. In some cases, separation may require ultrafiltration. Very fine powders may also clog or poison the reactors or the autoclaves employed in the chemical reaction. The support material may also pulverize upon stirring. Further, catalyst particles on support materials tend to aggregate upon use to form larger particles having smaller surface area and, hence, lower activity, ultimately resulting in catalyst deactivation after prolonged use. Metal leaching from the catalyst to the reaction solution may also represent a serious problem in terms of contamination of products for the fine chemical industry, (pharmaceutical, perfumery)
Due to the abovementioned reasons, most supported MNP-based catalysts are difficultly adaptable into efficient reactors suitable for the large scale-production of fine-chemicals.
One of the inventors of the present invention described new inorganic/polymeric hybrid membranes in Electrochemistry, 72, 111-116 (2004), JP 3889605, U.S. Pat. No. 7,101,638, JP 3856699. The membranes consist of a hybrid compound of inorganic oxides and polyvinyl alcohol (PVA), in which the inorganic oxides are chemically combined with PVA through its hydroxyl groups. These materials are produced by simple processes in an aqueous solution, in which salts of inorganic oxides are neutralized by acid in the condition that PVA co-exists. By this method, the nascent and active inorganic oxides generated by neutralization combine and hybridize with PVA to form the hybrid compound. The hybrid compounds are distinguished from mixtures of inorganic oxides and PVA, that is, their chemical properties are remarkably changed from their raw materials. For example, once hybridized materials are insoluble in any solvents including hot water in spite of being made from PVA soluble in water.
These membranes have been developed for application as proton conductive solid electrolytes, especially in fuel cells. Accordingly, they have high chemical resistance against oxidation, reduction and radical attack as well as high thermal resistance. In that kind of electrolyte, protons are carried by using water molecules, as membranes are able to absorb water. In these hybrid membranes, inorganic oxides are dispersed as a very fine (nano-sized) particle, because PVA prevents the inorganic oxide from growing to a large size particle during the synthesis process of the hybrid compound.
No literature data are known in which the above membranes were used as support for MNP-based catalysts. The inventors of the present application disclosed these kinds of hybrid membranes as support material for molecular catalysts in PCT/JP2010/056288, wherein the immobilized molecular catalysts were limited to molecular metal complex, and not MNPs.
Catalytic membranes based on purely organic polymers embedding metal nanoparticles were previously described in the literature which do not contain any inorganic components, however: Adv. Synth. Catal. 350, 1241-1247 (2008), Catal. Today 104, 305-312 (2005) and Ind. Eng. Chem. Res. 44, 9064-9070 (2005) describe catalytic membranes based on Pd and Au NPs into polyacrylic acid and polyvinylpyrrolidone for use in hydrogenation and oxidation reactions; Chem. Mat. 17, 301-307 (2005) describes polyethylenimine and polyacrylic acid-based membranes containing Pd NPs for the catalytic hydrogenation of allylic alcohols; Water Res. 42, 4656-4664 (2008) describes Pd/Fe NPs into Polyvinylidene fluoride-based membranes for the catalytic dechlorination of trichloroacetic acid.